Treatment of ferrous metal



Unitd States Patnt O 3,382,110 TREATMENT OF FERROUS METAL Robert Lozano, Hammond, (Joy S. Ham, Munster, Charles C. Marshall, Gary, and Jack E. Joyce, Chesterton, ind, assignors to Inland Steel Company, Chicago, 111., a corporation of Delaware No Drawing. Continuation-impart of application Ser. No. 287,138, June 11, 1963. This application Oct. 27, 1966, Ser. No. 589,854

3 Claims. (Cl. 1486.15)

ABSTRACT OF THE DISCLOSURE A process for controlling the rate of pickling of the surface of a strip of low carbon ferrous metal, such as enameling iron, by contacting the surface of the metal with a phosphorus-containing solution having a pH below 7 to form thereon a surface film containing phosphorus, and heating the surface film to an elevated temperature in a reducing atmosphere to form a diffused surface layer containing phosphorus.

The present application is a continuation-inpart of applicants co-pending U. S. patent application Ser. No. 287,138, filed June 11, 1963 now issued as United States Patent No. 3,308,042.

The present invention relates generally to an improved method of controlling the reactivity of the surface of steel and more particularly to a method of controlling the pickle lag values of a steel surface.

When a steel strip is subjected to a rate of pickling test, it has been found that many steels will be attacked by the pickling acid at varying rates during the pickling test. Many steels, for example, display an initial period in the early part of the pickling test in which the steel is attacked at a relatively slower rate, with the steel below the outer layer utilimately exhibiting a substantially different linear or constant final rate of pickling. The rate of pickling can be determined either by measurement of the weight loss or the amount of hydrogen gas evolved. The initial period of low rate of pickling or of increasing rate of weight loss during pickling is called the pickle lag period. The quantity of surface metal or surface skin which is removed during the pickle lag period (i.e. until the rate of weight loss becomes substantially constant) is designated as the pickle lag layer. Generally speaking, the depth of the pickle lag layer or skin as it is sometimes called may be from about .0001 inch to about -.001 inch, as calculated from the quantity of metal re- 6 moved during pickle lag tests. The latter skin or pickle lag layer was first described by E. L. Koehler in Transactions of the ASM, vol. 44 (1952), 1076.

As described in US. Patent No. 2,872,353, it is im portant in order to insure good adherence of a vitreous enamel coating on a steel strip that any pickle lag layer or skin be first removed from the steel surface before enameling. It will therefore be evident that the surface of enameling steel, particularly when the steel is desired in an etched condition for improved paint or coating adherence, should preferably exhibit a rate of pickling greater than that of the underlying steel base in order to minimize the time required for removing the surface layer of metal and thereby reduce processing costs.

As disclosed in our co-pending US. application Ser. No. 287,138, however, it has been found that improved electrolytic tin plate can be produced when the surface of the metal to be electrolytically tinned does not have a high pickle lag and preferably has a rate of pickling which approximates the rate of pickling of the underlying base metal (i.e. substantially zero pickle lag).

3,382,1l0 Patented May 7, 1968 Therefore, it is an object of the present invention to provide an improved method of controlling the reactivity of a ferrous metal surface which is exposed to an acid pickling solution.

It is also an object of the present invention to provide a method of producing a ferrous metal having a surface with a rate of pickle which is higher, lower, or the same as the rate of pickle of the underlying base metal.

Other objects of the present invention will be apparent from the detailed description and claims to follow.

It has been discovered that the reactivity to a ferrous metal acidic pickling solution or the pickle lag value exhibited by the surface of a metal strip, such as a strip of enameling iron or a strip of black plate suitable for electrolytic tin plating, can be controlled by treating the steel strip under controlled conditions so as to provide a thin diffused layer containing an element of the group consisting of phosphorus, sulfur or nitrogen, and particularly good results have been achieved by providing a very thin diffused surface layer containing phosphorus.

In the preferred process of the present invention phosphorus in the form of a compound of phosphorus which is chemically reactive with the metal surface is brought into contact with the metal surface where it forms thereon a thin film or coating containing phosphorus, and thereafter the phosphorus-containing film or coating is diffused into the surface of the metal by heating the coating metal in a controlled reducing atmosphere.

In the preferred embodiment of the invention the chemical treatment step is carried out by incorporating a small amount of a phosphoric acid or a mixture of phosphoric acid and a water soluble phosphate salt, such as tetrasodium pyrophosphate or sodium acid phosphate, preferably through the final preannealing rinse water sprays, to provide a dilute acidic phosphate treating bath immediately following the conventional preannealing cleaning treatment and without requiring any other alterations in the metal composition or in the manner of treatment of the said strip.

The phosphorus-containing chemical bath in which the strip is immersed or otherwise contacted preferably comprises a dilute phosphoric acid solution having a pH below 7 and preferably not substantially lower than about pH 2. The preferred range of pH values is from about pH 2 to about pH 6. The aqueous phosphorus-containing solution normally contains between about 500 and 1500 ppm. phosphate ion and preferably below 1000 ppm. On a percentage basis the phosphorus concentration has a range of between about .05% and 0.2% by weight and preferably below 0.1%. The chemical treatment with the herein described phosphate ion-containing solutions provides on the surface of the strips treated therewith an amount of phosphorus equivalent to between about .00015 and .00035 gram P0 per square foot, as shown by analysis of several phosphate treated annealed products. The control strip of low carbon steel from the same melt as the treated test strips and processed in the same way ex cept that the strip was not exposed to the phosphate'ion containing solution, when analyzed in the same manner for phosphorus, had no measurable phosphorus in the surface thereof.

While the chemical treating solution is conveniently provided by means of adding phosphoric acid (H PO or one of the other phosphoric or phosphorous acids to the continuous anneal prefurnace final water rinse, it is also possible to add to the rinse water a water soluble phosphate salt, such as sodium acid phosphate (Na PO or tetrasodium pyrophosphate (Na P O along with suflicient phosphoric acid or other acid of phosphorus to bring the pH of the bath within a range of pH 2 to 6 and preferably to pH 3 to 5. It is further contemplated that a non-phosphoric acid could be used to adjust the pH of the phosphate-containing solution, such as nitric acid or sulfuric acid. When an alkaline phosphate salt is used with a phosphorus acid, a higher total phosphate concentration is possible without exceeding the desired pH range. Thus, a total phosphate-ion concentration of 1500 ppm. can be present in the treating solution when comprised of tetrasodium pyrophosphate and sufficient phosphoric acid to adjust the pH to below 7.

1f preferred, it is possible to treat the steel strip with the acid phosphate solution or equivalent phosphorus compound at a point prior to annealing other than immediately before the normal annealing step. For example, the strip may be immersed or sprayed with an active acidic phosphorus solution or composition in the final water rinse just after the first acid pickling bath treatment and immediately before the cold rolling mill treatment.

As a further modification of the present invention, it is possible to provide the required diffused coating or film containing phosphorus on the surface of the metal strip to be electrolytically tin plated by contacting the metal strip with a phosphorus-containing gas under conditions which form a substantially continuous and uniform coating or film of phosphorus or phosphorus compound on the surface of the strip and diffusing the film into the surface of the strip prior to electrolytically tin plating For example, the strip while heated at a temperature of about 1300 F., as when being annealed, can be contacted by phosphine (PH gas which decomposes on the surface of the strip to provide a film of elemental phosphorus and which diffuses into the surface of the strip at the annealing temperature. Other atmospheres or gases containing phosphorus which provide a phosphorus-containing surface film diffusable into the steel strip can also be used in place of phosphine.

Following the chemical treatment of the steel base strip it is essential that the strip having the phosphorus-containing film or an iron phosphate film on the surface thereof be heated to a moderately high temperature in a reducing atmosphere to effect diffusion of the film into the surface of the strip. The required heating in a reducing atmosphere is most conveniently carried out by passing the strip at the normal rate of travel (between approximately 800 and 1240 ft. per min.) continuously through a standard continuous annealing furnace having a normal hydrogennitrogen reducing atmosphere (5% H -95% N and at the normal operating temperature of around 12004500 F. Other types of continuous annealing apparatus or equivalent apparatus can be used, such as an open coil annealing equipment or a continuous normalizer or similar equipment employing a heated reducing atmosphere. If desired, however, the coiled strip can be box annealed for a period of from about 6 to 48 hours in the presence of a reducing atmosphere, depending on the degree or depth of diffusion desired or the special properties re quired of the base metal strip.

While it is preferable that the surface coating formed from the acidic phosphorus solution be exposed to a reducing atmosphere at an elevated temperature between about 1200 F, and 1500 F. for a period of about to 50 seconds, the strip can be heated longer at a lower temperature or for a shorter period at a higher temperature without adversely affecting the results. It is not necessary to have any particular reducing atmosphere in the annealing chamber and any of the commonly available reducing gases can be used, such as hydrogen-nitrogen, dissociated ammonia, and the like. Nor is it essential that the atmosphere have a particular dew point. The temperature, time of treatment, and atmosphere in the annealing chamber is in each instance, however, regulated and controlled so as to effect diffusion of the phosphorus-containing film into the surface of the strip to form an integral continuous phosphorus-containing diffused film while the said strip remains in the annealing chamber. The phosphorus film preferably is diffused into the strip to a depth greater than the depth of surfaces, scratches or markings which are formed in the strip following annealing and prior to tin plating. The depth of the diffused layer is primarily de pendent on the time the strip is held at the elevated diffusing temperature.

In practicing the preferred embodiment of the present invention, a hot rolled band about 0.1 inch thick is continuously immersed in a conventional sulfuric acid pickle bath and passed through a tandem rolling mill to provide a full hard, low carbon, cold rolled steel strip which generally has a thickness of between about .005 to .02 inch and is referred to as black plate. The steel strip traveling at a rate of between about 800 and 1250 feet per minute is subjected to a continuous preannealing chemical cleaning treatment at a cleaning station to remove rolling oils and other surface contamination. In the preannealing continuous chemical cleaning treatment, the strip is continuously dipped in an alkaline cleaning bath, then passed through an electrolytic alkaline cleaning solution, and finally through a scrubber where the surface of the strip is contacted by brushes and sprayed to remove any firmly adhering particles. Each of the foregoing cleaning steps can be any of the conventional types presently used in the art,

The cleaned steel strip which is normally given a final preannealin g water rinse following the foregoing chemical cleaning treatment before passing through a continuous annealing furnace is, in accordance with the preferred embodiment of the present invention, next chemically treated at a treating station by spraying, dipping, wiping, or the like with an aqueous treating bath comprising a dilute acidic aqueous solution of a phosphorus compound, preferably containing a phosphate ion or an equivalent phosphorus oxide ion, to provide on the surface of the strip an r integral thin film or coating containing phosphorous, and

with said phosphorus compound being ditfusible into the surface of the steel strip when subjected to elevated temperatures in a reducing atmosphere.

The steel strip preferably has its surface uniformly coated with a film of an acidic phosphate ion-containing solution by immersing the strip in the treating bath, passing the strip between conventional squeegee rolls which remove excess solution, and rapidly drying the strip by means of conventional gas dryers to form the dry film or coating on the surface of the steel strip. The steel strip with the coating on the surface can be passed directly into the continuous annealing furnace, or the treated strip can be placed in a box annealing chamber of any conventional type, if it is necessary to provide a deeper diffused phosphorus-containing coating or special metal properties are desired. If desired, however, the annealing can take place at a subsequent time and the strips can be coiled for storage.

In the preferred embodiment the steel strip having a thin dry film or coating of a phosphorus compound on the surface thereof is continuously passed through a conventional continuous annealing furnace having therein a reducing atmosphere, such as 5% hydrogen and nitrogen, in which the strip remains for the usual period of between about 30 and 50 seconds and during which period the phosphorus compound is reduced and diffused into the surface of the steel strip to form a diffused phosphorus-containing surface layer.

When the steel strip having the phosphorus-containing film diffused into the surface thereof is to be electrolytically coated with tin, the annealed strip is temper rolled and then cleaned and passed through a conventional sulfuric electrolytic pickle bath normally containing about 3% by volume sulfuric acid to remove any surface oxides which have formed subsequent to annealing or may be passed directly to electrolytic acid tinning, if no objectionable oxides or contamination are present on the surface of the strip. However, any conventional acid electrolytic tinning line can be used to tin plate the treated annealed strip, such as the preferred Ferrostan line which normally includes an electrolytic alkaline phosphate dip, an electrolytic sulfuric acid pickle, cleaning by one or more scrubbers, immersion in an acid tin plating bath contained in a plurality of electrolytic acid tin plating tanks, followed by fusion and chemical treatment, such as a cathodic dichromate treatment. The resultant acid tin plated strip is thereafter generally oiled in the usual manner and coiled into sheets, if desired.

The following specific examples are for the purpose of further illustrating the present invention but should in no way be construed as limiting the invention to the particular materials or conditions specified.

Example 1 A low carbon steel strip of the type generally designated black plate and having a thickness of .01 inch, which is well suited for the production of electrolytic acid tin plate and adapted for use in the manufacture of tin cans for packing fruit and juices, while moving at a rate of about 1000 feet per minute was thoroughly cleaned by a conventional pre-annealing cleaning treatment comprising an alkaline dip, an electrolytic cathodic cleaning step and scrubbing and immediately thereafter was continuously passed through a phosphoric acid final pre-annealing aqueous rinse solution containing sufficient phosphoric acid (H PO to provide a pH ranging between 2.2 and 3.1. The phosphoric acid was introduced into the final preannealing rinse water sprays which were equipped with proportioning pumps to add continuously the required amount of phosphoric acid to maintain the pH between about 2 and 3. The steel strip at a temperature of about 180 F. remained in contact with the dilute phosphoric acid solution also at a temperature of about 180 F. for a period of between about 1 and 3 seconds. After passing the strip through rolls to form a uniform film and drying the strip by a conventonal dryer, the strip was then passed directly into a continuous annealing furnace maintained at a temperature of about 1350 F. In the annealing furnace a reducing atmosphere was maintained which comprised about 4% hydrogen and 95% nitrogen. The dew point of the annealing atmosphere was maintained between plus 30 F. and plus 45 F. The steel strip having the phosphate ion-containing film on the surface thereof on emerging from the annealing furnace had a phosphorus-containing layer diffused into the surface thereof, exhibited a pickle lag of from 0 to +4 seconds, whereas initially the pickle lag value for the strip without the phosphate treatment was to +16 seconds.

Example 2 A low carbon strip of black plate as in Example 1 traveling at a rate of about 1000 feet per minute after a conventional pre-annealing cleaning treatment was continuously passed through a dilute phosphoric acid final pre-annealing rinse solution having a total phosphate ion concentration of 500 p.p.m. (parts per million) and a pH of 3.4 with said acid being introduced through the final rinse sprays. Following continuous annealing as in Example 1, the steel strip exhibited pickle lag values of 0 second.

Example 3 A clean low carbon steel strip as in Example 1 was continuously passed through a pre-annealing final water rinse to which phosphoric acid had been directly added in an amount sufficient to provide a total phosphate ion (P0 concentration of 900 p.p.m. and a pH of about 3.5, followed by the standard annealing treatment as in Example 1 in which the hydrogen reducing atmosphere had a dew point of plus 45 F. The treated annealed steel strip exhibited a pickle lag of plus 6 seconds.

Example 4 A clean low carbon steel strip as in Example 1 was continuously passed through a pre-annealed water rinse solution containing sufiicient phosphoric acid to provide a total phosphate ion (P0 concentration of 1200 p.p.m. and a pH of 2.5. Following the standard annealing treatment in which the reducing hydrogen atmosphere exhibited a dew point of plus 39 F., the treated annealed steel strip exhibited pickle lag values of between 0 and 3 seconds, whereas the strip before the phosphate treatment had pickle lag values between 12 and 16 seconds.

Example 5 A low carbon steel strip as in Example 1 was continuously passed at a rate of about 1000 feet per second through a pre-annealing final water rinse bath following the conventional pre-annealing continuous cleaning with said bath having directly added thereto sufficient phosphoric acid to provide a total phosphate ion (P0 concentration of about 1400 p.p.m. and a pH of about 1.8 and a small amount of a wetting agent.

Example 6 A clean low carbon steel strip as in Example 1 was continuously passed through a pre-annealing final water rinse tank solution containing the usual volume of water and to which was added directly without using the rinse sprays five liters of phosphoric acid and 25 pounds of tetrasodium pyrophosphate to provide a total phosphate ion (P0 concentration of between 900 and 1400 p.p.m. and provide an initial pH of 3.3. After an endless steel strip was continuously passed through the phosphate solution for 2 hours 15 minutes without further addition of phosphoric acid or phosphate salt, the pH of the solution rose to pH 5.6. The phosphate treated strip was then continuously annealed, as in Example 1. The portion of the steel strip which was treated when the solution had a pH of 3.3 exhibited a pickle lag after annealing of minus 3 seconds, and the portion which was treated when the solution had a pH of 5.6 exhibited a pickle lag of plus 6 seconds.

In the foregoing specific examples particularly good results were obtained when the final pre-annealing rinse solution had a total ion phosphate concentration below 1000 p.p.m. and with a pH below 6. In certain of the phosphate chemical treating runs, particularly where the total phosphate ion concentration was about 1000 p.p.m. and no precautions were taken to remove any surface solids, a white precipitate which has been identified as a ferric phosphate, probably ferric orthophosphate or ferric pyrophosphate, increased in concentration during the chemical treatment of the strip and tended to remain on the surface of the strip as a visible deposit. When the steel strip was annealed with a significant excess amount of ferric phosphate on the surface, as when there was no final rinse sprays used to flush the strip as the strip left the phosphate treating bath, the electrolytic acid tin plate produced from the annealed strip having the ferric phosphate on the surface assumes a dull appearance which is objectionable for some uses of tin plate. The foregoing objectionable result was corrected, however, by subjecting the annealed strips to a more intensive pickling treatment immediately prior to electrolytic tinning, as in Example 6.

It is postulated that in the foregoing treatment the acid phosphate ion or equivalent metal ion may react with the ion in the surface of the strip to form an iron phosphate surface film which may thereafter be partially or completely reduced by the reducing atmosphere and annealing treatment to form a diffused layer of iron phosphide. It is also possible, however, that the phosphate ion or other reactive phosphorus compound may be directly reduced to elemental phosphorus which might then be diffused into the surface of the steel strip to form an iron-phosphorus alloy during the annealing treatment. Whatever the precise chemical form of the phosphorus in the surface of the annealed strip, the acid tin plate after conventional surface fusion of the tin to flow brighten has a normal amount of iron-tin alloy formed between the surface of the steel strip and the free tin layer. Thus, the iron phosphate which may be reduced to iron phosphide or iron phosphorus alloy diffused in the surface of the strip does not appear to inhibit the formation of the conventional iron-tin alloy layer, and the improved results achieved by the present invention would not appear to be due to any change in amount of the iron-tin alloy layer. The present invention, however, is not dependent on any theory of operation, since the improved results are achieved by employing the herein disclosed combination of treating steps in the production of electrolytic tin plate, and particularly acid tin plate.

From the preceding examples and discussion it will also be evident that the present invention provides an improved method of controlling the pickle lag of a steel strip to any desired value below the normal pickle lag values of a ferrous metal strip. In general the decrease in the pickle lag is a function of the acidity of the phosphate solution (see Example 6). In some instances where the pH of the phosphate solution is relatively low or the P concentration relatively high, there is an actual increase in the rate of pickling above the constant rate of pickle of the base metal, and the strip is designated as having a negative or minus pickle lag. The strip has a positive or plus pickle lag when the strip has an initial rate of pickle slower than the constant rate of pickle of the base metal. The herein disclosed chemical treatment provides a convenient means for adjusting or controlling the pickle lag of a metal strip to a desired plus, zero, or negative pickle lag value, where the art considers this necessary or helpful to provide a more reactive base or an improved base for any surface coating to be applied on the strip. Thus, if a surface layer more reactive than base metal is desired, the treating solution used in the present process should have a pH below pH 4 and preferably between about pH 2 and pH 4. And, if a surface layer having about the same reactivity as the base metal is desired, the treating solution used in the present process should have a pH between about pH 4 and pH 5, and preferably about pH 4.5. It has also been observed that a negative or minus pickle lag value of a treated strip can be increased, if desired, to a zero or positive (-1-) pickle lag value by subjecting the here- 8 in treated annealed strip to a further pickling treatment with sulfuric acid.

Others may practice the invention in any of the numerous ways which are suggested to one skilled in the art by this disclosure, and all such practices of invention are considered to be a part hereof which fall within the scope of the appended claims.

We claim:

1. A process of treating a surface of a strip of low carbon ferrous metal to control the rate of pickling of said surface relative to the rate of pickling of said ferrous metal which comprises contacting said surface with a phosphorus treating fiuid containing phosphate ion in a concentration between about 500 to 1500 parts per million and having pH of between about pH 2 and pH 6 which forms a surface film containing phosphate and wherein the said pH and said concentration of phosphate ion in said treating fluid are correlated to effect changing the rate of pickling of said surface after diffusion of said phosphorus into said surface, and heating said surface film to a temperature of at least about 1200 F. in a reducing atmosphere comprised essentially of nitrogen and hydrogen until forming a diffused layer at said surface containing phosphorus equivalent to between about .00015 and .00035 gram P0 per square foot.

2. A process as in claim 1, wherein said treating fluid has a pH between about pH 4 and pH 5, whereby said steel surface has a pickle lag value of about 0.

3. A process as in claim 1, wherein said treating fluid has a pH below about pH 4, whereby said steel surface has a negative pickle lag value.

References Cited UNITED STATES PATENTS 2,071,533 2/1937 Ihrig 148-6 2,310,451 2/1943 Marshall 1486.15 3,193,417 7/1965 Kopchak 148--36 X OTHER REFERENCES Bablik: Galvanizing (Hot Dip), 3rd Ed, 1950, E and R. N. Spoon Ltd., pp. 56, 57, 62 and 63.

Spring: Metal Finishing, September 1965, pp. 71-75.

RALPH S. KENDALL, Primary Examiner. 

